Diffractive optical elements (hereinafter referred to as DOEs) provide arbitrary optical powers and have anomalous dispersion characteristics that can effectively reduce chromatic aberration of a refractive optical system.
However, the DOEs are nearly always provided with reduced optical powers to be used in optical systems for multicolor lights so as to correct the chromatic aberration well. This is because a dispersion of the DOE is extremely larger than that of refraction and therefore a DOE having an optical power significantly contributing to image-formation increases differences of diffraction powers for various wavelengths, which increases chromatic aberration generated by the DOE. Thus, performances of the DOEs are not utilized enough.
Japanese Patent No. 3966303 discloses a pickup lens having on its each surface binary step with different heights. This pickup lens sets the height of binary steps on one surface to a value equal to an integral multiple of a wavelength that is not desired to be diffracted through the binary steps and to a value different from an integral multiple of a wavelength that is desired to be diffracted therethrough, thereby diffracting only light of the desired wavelength.
Moreover, Japanese Patent Laid-Open No. 9-189892 discloses a displaying optical system including a liquid crystal DOE. The displaying optical system performs high-speed time division switching of a wavelength of light from a light source such as R→G→B→R→ . . . , and switches parameters of the liquid crystal DOE in synchronization with the time division switching, thereby suppressing generation of aberration.
Description will be made of an example of a transmissive DOE in which concentric annular zones are formed on a transparent flat substrate whose refractive index n(λd) is 1.5168 (λd=587.56 nm). When a diffraction order of the DOE is +1st order and a focal length thereof is 50 mm, if an entrance pupil is disposed coaxially with the annular zones and a diameter of the entrance pupil is 5 mm, a longitudinal chromatic aberration of R-B (λR=640 nm and λB=480 nm) increases to 15.704 mm. In a case of a refractive lens having a refractive index identical to that of the DOE and a focal length of 50 mm, a curvature radius is −28.63 mm and the longitudinal chromatic aberration of R-B is 0.775 mm.
Furthermore, description will be made of an example of a reflective DOE that converts an incident angle of 25° into a reflection angle of 60° and whose diffraction order is +1st order and focal length is 50 mm. In this DOE, the longitudinal chromatic aberration of R-B increases to 40 mm or more.
These descriptions were made of the cases where the DOE is used alone. However, in a case where the DOE having a strong power is used in an optical system including lenses or mirrors, an extremely large chromatic aberration is generated due to diffraction by the DOE, which may prevent formation of the optical system.
The DOEs disclosed in Japanese Patent No. 3966303 and Japanese Patent Laid-Open No. 9-189892 may solve the above-described problem. However, the DOE disclosed in Japanese Patent No. 3966303 is a multi-level zone plate DOE, which may obtain an insufficient diffraction efficiency. Further, the liquid crystal DOE disclosed in Japanese Patent Laid-Open No. 9-189892 involves a problem that accuracy of an annular zone interval depends on a size of a pixel cell and a problem that temporal responsiveness thereof cannot be sufficiently improved.